Molecular sieve abstract
Methods for forming membranes of semiconductive material incorporating
molecular sieve material therein can involve sol gel techniques
and suction techniques. The semiconductors, such as metal oxides,
sulfides or carbides have photoactivity and are useful in photocatalytic
reactions. An example of such a membrane is titanium oxide including
zeolite molecular sieve incorporated therein. The membranes can
be used to perform useful chemical reactions such as the mineralization
of organic chemicals in the presence of light. For example, many
toxic organic chemicals can be converted to useful or benign products
by contacting those chemicals with active metal oxide-molecular
sieve membranes in accordance with the invention and illuminating
the membranes with light of a suitable wavelength.
Molecular sieve claims
What is claimed is:
1. A membrane for affecting the composition of material coming
in contact with the membrane, comprising:
an inert porous substrate having a pore size from about 2.5 to
50 microns and semiconductor material including TiO.sub.2 supported
by the porous substrate, the semiconductor material including a
photoactive polymeric metal/semimetal oxide matrix and molecular
sieve material impregnated within the matrix, the molecular sieve
material including zeolitic material having pore size from about
3 to 12 angstroms.
2. The membrane of claim 1 wherein the membrane includes metal
ions.
3. The membrane of claim 1 wherein the membrane includes transition
metal ions.
4. The membrane of claim 2 wherein the metal ions are Cu.sup.2+
ions.
5. The membrane of claim 1 wherein the membrane includes transition
metal ions.
6. The membrane of claim 5 wherein the metal ions are Cu.sup.2+
ions.
7. The membrane of claim 1 wherein the molecular sieve is selected
from the group consisting of silicalite, ZSM-5 NaA, Y zeolite,
mordenite, ETS-10 and combinations thereof.
8. The membrane of claim 1 wherein the molecular sieve is silicalite.
9. The membrane of claim 1 wherein the molecular sieve includes
titanium.
10. The membrane of claim 1 wherein the semiconductor includes
TiO.sub.2 and the membrane also includes SiO.sub.2.
11. The membrane of claim 1 wherein the substrate includes material
selected from the group consisting of glass, fused silica, quartz
and ceramic material.
12. A method of forming an inorganic membrane, comprising:
impregnating a porous substrate with molecular sieve particles
and exposing the substrate with a titanium alkoxide to form a film
of titania on the surface of the impregnated substrate.
13. The method of claim 12 wherein the molecular sieve is zeolite
material.
14. The method of claim 12 wherein the alkoxide is titanium isopropoxide.
15. The method of claim 12 wherein the porous substrate impregnated
with molecular sieve particles is reacted with a liquid solution
to form titania on the impregnated substrate.
16. The method of claim 14 wherein the impregnated substrate is
immersed in a neat titanium isopropoxide solution.
17. The method of claim 14 wherein the molecular sieve includes
zeolite material.
18. The method of claim 12 wherein the alkoxide is applied from
the vapor phase.
19. The method of claim 12 wherein metal ions are incorporated
in the membrane.
20. The method of claim 12 wherein Cu.sup.2+ ions are incorporated
in the membrane.
Molecular sieve description
BACKGROUND OF THE INVENTION
This invention relates generally to inorganic membranes. Such membranes
are particularly well suited for conducting chemical reactions including
the mineralization of certain organic chemicals, the separation
of mixtures and the treatment of municipal and industrial waste.
The use of membranes to separate mixtures and catalyze chemical
reactions is becoming an important chemical technique. Membrane
separations tend to require less energy than competing techniques
such as distillations. Furthermore, the use of membranes in chemical
processes can be less costly and more simple to implement than other
techniques.
Many techniques have been employed to form inorganic membranes.
Examples of these include laser drilling, slip casting, track etching,
anodic oxidation and the use of sol gel technology. Sol gels are
formed through the acid or basic catalysis of the hydrolysis of
metal or semi-metal alkoxides. The gel can be dried and fired to
yield amorphous and ceramic-type membrane materials. The use of
sol gel technology to prepare titania ceramic membranes is described
in PCT Patent No. WO 8900983 and WO 8900985 the contents of which
are incorporated herein by reference.
Molecular sieve material has been used to effect separations and
to catalyze chemical reactions. Molecular sieves are a class of
materials which contain pores and/or cages with a size similar to
that of many organic molecules. Accordingly, molecular sieves can
differentiate and separate organic molecules based on the size of
the molecules.
Semiconductor particles including metal oxides, sulfides and carbides
have been used to catalyze many important reactions. These semiconductor
materials have photoactivity and are well suited as catalysts in
photochemical reactions. However, these reactions are typically
conducted in a liquid suspension of the semiconductor particles.
Municipal and industrial waste management has become one of the
most serious and urgent problems facing modern society. The most
commonly employed solutions involve either land disposal or burning
of organic waste material in either open air or closed system incinerators.
However, these methods are becoming disfavored because of the limited
availability of landfill sites and the high energy costs associated
with incineration as well as the problem of dealing with the gases
and solid incineration by-products.
Accordingly, it is desirable to produce improved membranes for
carrying out important industrial processes. For example, it would
be particularly desirable to develop membranes for treating municipal
and industrial waste.
SUMMARY OF THE INVENTION
Generally speaking, in accordance with the invention, methods are
provided for forming membranes of semiconductive materials incorporating
molecular sieve materials, such as zeolites, therein. The semiconductors,
such as metal oxides, sulfides or carbides are photoactive and are
useful in photocatalytic reactions. Methods for forming the membranes
can involve sol gel techniques and surface reaction techniques.
An example of such a membrane is titanium oxide including a zeolite
molecular sieve incorporated therein.
Membranes in accordance with the invention can be used to perform
useful chemical reactions such as the mineralization of organic
chemicals in the presence of light. For example, many toxic organic
chemicals can be converted to useful or benign products by contacting
those chemicals with active metal oxide-molecular sieve membranes
in accordance with the invention and illuminating the membranes
with light of a suitable wavelength.
Accordingly, it is an object of the invention to provide an improved
inorganic membrane.
Another object of the invention is to provide an improved method
of forming a membrane including both metal oxide and molecular sieve
material.
A further object of the invention is to provide an improved method
of mineralizing toxic organic chemicals.
Yet another object of the invention is to increase the photoefficiency
of conventional membranes.
Still other objects and advantages of the invention will in part
be obvious and will in part be apparent from the specification.
The invention accordingly comprises the several steps and the relation
of one or more of such steps with respect to each of the others
and the membrane possessing the features, properties and the relation
of constituents which are exemplified in the following detailed
disclosure and the scope of the invention will be indicated in the
claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Membranes in accordance with the invention can include active metal
oxides, metal sulfides and carbides that are photoactive together
with a molecular sieve material. Many of these semiconductive particles
have found important uses in catalyzing chemical reactions. Semiconductors
are useful in conducting photochemical reactions. Molecular sieves
are useful both for conducting separations and as catalysts. It
has been determined that extremely useful membranes can be formed
by combining semiconductor material and molecular sieves in a single
membrane in intimate contact so that both materials can enhance
the effectiveness of the other.
Molecular sieves are a class of materials that have pores and/or
chambers that are similar in size to many organic molecules. Accordingly,
they can selectively adsorb molecules based on the size and shape
of the molecules. Zeolites are one family of molecular sieves. Zeolites
are crystalline materials that contain silicon oxides and aluminum
oxides formed with a three dimensional structure in which tetrahedra
of primarily SiO.sub.4 and AlO.sub.4 are crosslinked by sharing
oxygen atoms, whereby the ratio of Si to O atoms can be about 1:2.
Depending on the manner of bonding, the crystal structures can have
various ring sizes containing 8 10 12 or more metal/semi-metal
atoms. This ring structure leads to a crystal structure with pores
and chambers that are of a molecular size.
The zeolite can have the general formula:
where X is a trivalent element such as Al, B, Fe, Ga and combinations
thereof; Y is a tetravalent element such as Si, Ge, Ti and combinations
thereof; and n is at least 2.
The semiconductor portion of the membrane can include a photoactive
polymeric metal/semimetal oxide matrix and a molecular sieve can
be impregnated in the matrix. Suitable metal oxides include TiO.sub.2
WO.sub.3 ZnO, CdO, Fe.sub.2 O.sub.3 and SrTiO.sub.3. Certain sulfides
are also suitable, including CdS and ZnS. Carbides such as SiC can
also act as photocatalysts.
Titanium oxide (TiO.sub.2) or titania can be an important component
of a membrane in accordance with the invention. TiO.sub.2 can also
be combined with SiO.sub.2. Titania can be used for a variety of
processes and purposes. In addition to separations, titania can
also be used in various catalytic applications. Because titania
is a semiconductor, it can catalyze many chemical reactions upon
proper irradiation. Furthermore, catalytic applications using the
titania in the membrane form will offer advantages over conventional
slurry suspensions of titania. For example, the costly filtration,
resuspension and recirculation steps associated with conventional
titania slurry systems are eliminated when the desired reaction
is catalyzed with a titania membrane. Membranes including titania
can also be used as an efficient adsorbent or filter in addition
to being the catalyst for a chemical reaction.
A photocatalyst of TiO.sub.2 is useful in many important chemical
reactions. For example, TiO.sub.2 catalyzes the conversion of CH.sub.4
and NH.sub.3 to amino acids in water. H. Reiche, A. J. Bard, J.
Am. Chem. Soc. (101) 3137 (1979). Photocatalytic conversion of nitrogen
and water to ammonia can be carried out on iron doped TiO.sub.2.
G. N. Schrauzer, T. D. Guth, J. Am. Chem. Soc. (99) 7189 (1977).
The contents of these two articles are incorporated herein by reference.
TiO.sub.2 is also used in the photo-assisted water--gas shift reaction,
i.e., the reaction of carbon monoxide and water to yield carbon
dioxide and hydrogen gas.
TiO.sub.2 can also be important in artificial photosynthesis to
yield hydrogen fuel from photocatalytic decomposition of water or
biomass. Hydrocarbons can also be converted to oxygen-containing
compounds in the presence of TiO.sub.2 photocatalysts. However,
under conventional methods and with conventional forms of materials,
these TiO.sub.2 photocatalyzed reactions have exhibited a rather
low quantum yield of less than about 2%. This low quantum yield
is not fully satisfactory and has been a major limitation in the
commercial application of TiO.sub.2 photocatalysis.
Titania particles will be extremely useful in organic waste management.
For example, many toxic chemicals in waste streams can be mineralized
or otherwise rendered benign when illuminated with near-ultraviolet
light or simulated sunlight in the presence of semiconductor particles
such as titania. Titania can be photoactivated, it is chemically
stable and it is relatively inexpensive. Furthermore, it is a constituent
of many natural clays and is therefore environmentally friendly.
Light absorption at a semiconductor surface is analogous to the
adsorption of photons by an atom or molecule. Electron excitations
occur when the incident light energy equals or exceeds the energy
difference between the valence band and the conduction band. This
leads to the generation of free electrons and electron holes (vacancies).
The photogenerated holes at the surface of the semiconductor material
are available for use in oxidation reactions and the photogenerated
electrons can be used in reduction reactions.
A photoredox reaction at a semiconductor interface can involve
reactive hydroxyl radicals. It has been proposed that the mechanism
of generation of hydroxyl radicals is through the oxidation of H.sub.2
O or OH.sup.- by the holes. Hydroxyl radicals react with organic
molecules adsorbed on the semiconductor surface or dissolved in
the solution to yield the products. In addition to direct light
absorption by TiO.sub.2 metal ions and metal ion complexes can
absorb light and sensitize the photoreactions.
It has been discovered that the quantum yield of TiO.sub.2 photocatalyzed
reactions can be markedly increased by combining the TiO.sub.2 material
with a molecular sieve in the form of a membrane. Preferable wavelengths
of illuminating radiation are between 200 and 800 nm. The combination
provides a synergistic effect in which the reaction yields of the
combined materials exceed that for either material on its own on
a same weight basis. Molecular sieves can selectively adsorb molecules
based on the size and shape of the pores or chambers. If it is desired
to selectively adsorb organic molecules from aqueous solutions,
the molecular sieve should be both hydrophobic and organophilic.
In zeolites this can be achieved by increasing the Si/Al ratio to
a relatively high level. On the other hand, zeolites can also serve
as ion exchangers. Providing zeolites with a high ion-exchange capacity
is optimized by providing zeolites with relatively low Si/Al ratios.
Zeolites can be classified into small pore, medium pore and large
pore families. The pore sizes range from about 3 .ANG. to about
12 .ANG.. Zeolites can also include chambers present in the zeolite
framework with sizes up to about 13 .ANG.. Thus, different types
of zeolites are preferred for different purposes. For example, for
the adsorption of organic molecules, medium and large pore zeolites
with high Si/Al ratios, such as silicalite, mordenite and high silica
NaY are preferred. For ion exchange purposes, zeolites with relatively
low Si/Al ratios, such as NaA are preferred.
In general, metal oxide membranes containing molecular sieve material
can be prepared with a process involving sol gel technology. First,
molecular sieve particles are incorporated in a porous substrate.
Then a metal alkoxide is hydrolyzed in water catalyzed with an acid
solution. Peptization of the liquid results in the formation of
a colloidal suspension referred to as a sol. As an example of the
foregoing, a titanium alkoxide can be hydrolyzed in water, catalyzed
with an acid solution and peptized with nitric acid to result in
the sol. The sol is then allowed to pass through the porous substrate
that includes molecular sieve and is incorporated therein to yield
a substrate-sieve-sol combination. The sol in the substrate is then
dried and sintered to form a molecular sieve-metal oxide membrane.
To incorporate zeolite particles in the supporting substrate of
the membrane, the zeolite particles can be suspended in water and
then drawn through a porous substrate with vacuum to trap the zeolite
particles in the porous substrate. The sol can then be allowed to
percolate through the zeolite impregnated porous substrate such
as by osmotic pressure for example. The sol can then be evaporated
to yield a semi-solid or gel. The semi-solid or gel can be sintered
by firing at a high temperature to form a membrane including metal
oxide and zeolite material.
There are five primary variables that should be controlled carefully
to achieve the most desirable results. The first is the ratio of
water to titanium. This ratio determines the concentration of titanium
hydroxide formed in the hydrolysis which affects the gel-like properties
of the material. The second variable is the ratio of acid catalyst
to titanium which can affect the pH and the stability and polymer
qualities of the gel. The third is the pH of the colloidal mixture
which can also affect the polymer quality of the gel. The fourth
variable is the sintering temperature which can affect the pore
size of the membrane and crystal phase of the membrane. The fifth
variable is the concentration of the zeolite particles in the suspensions
which affects the percent loading of zeolite in the resulting membrane.
A preferred alkoxide starting material is titanium tetraisopropoxide
Ti(OiPr).sub.4 and hydrochloric acid is a preferred catalyst for
the hydrolysis. A suitable molar concentration for conducting the
sol gel formation process is 1 Ti(OiPr).sub.4 :100 H.sub.2 O:0.3
HCl. Acceptable ratios of titanium tetraisopropoxide to water are
between 10 and 500 more preferably between 100 and 350. The ratio
of H.sup.+ to titanium tetraisopropoxide can be between 0.001 and
1 more preferably between 0.1 and 0.8. Although hydrochloric acid
is a preferred inorganic acid, the reaction can typically be catalyzed
with any organic or inorganic acid.
The titanium tetraisopropoxide should be added to the water with
vigorous stirring and hydrolysis will proceed to result in a polymeric
titanium hydroxide precipitate. The titanium hydroxide precipitate
can then be peptized with the amount of nitric acid necessary to
bring the pH of the sol below about 3 and the solution is heated
to a temperature between 50.degree. and 100.degree. C., more preferably
between 70.degree. and 80.degree. C. The solution can also be sonicated.
This converts the precipitate into a highly dispersed and stable
colloidal suspension.
The hydrolysis of titanium isopropoxide can also be carried out
in an alcohol solution. The variables influencing the hydrolysis
process in an alcohol solution include the order of mixing of reagents;
the ratio of alcohol to titanium isopropoxide; the type of alcohol,
such as isopropanol; and the ratio of H.sub.2 O to titanium isopropoxide.
These variables affect the gel and polymer properties of the sol.
Titanium isopropoxide has four hydrolyzable alkoxy groups. Thus,
the ratio of H.sub.2 O to titanium isopropoxide in alcohol-based
hydrolysis should be in the range of 1:1 to 16:1 more preferably
between 2:1 and 4:1. An acceptable molar composition for the alcohol-based
procedure is 1 Ti(OiPr).sub.4 :25 iPrOH:3.5 H.sub.2 O:0.08 HCl.
The ratio of iPrOH to titanium isopropoxide should be at least 15:1
more preferably between 25:1 and 50:1. The ratio of H.sup.+ to titanium
isopropoxide is preferably from about 0.001:1 to 0.5:1 more preferably
from 0.005:1 to 0.1:1.
It has been determined that the order of mixing has significant
effects on the viability of the gels obtained. The titanium isopropoxide
should first be diluted in anhydrous alcohol. The acid catalyst,
HCl and H.sub.2 O are then diluted in the remaining alcohol. The
HCl--H.sub.2 O alcohol solution should then be added to the titanium
isopropoxide alcohol solution, with stirring, to provide an acceptable
titania colloidal suspension.
A preferred method of loading the molecular sieve or zeolite particles
on the membrane is to form a suspension of fine zeolite particles
in water. The concentration of the particles in the suspension will
determine the amount of zeolite material in the resulting membrane.
A preferred substrate for the membrane is porous inert material,
such as inorganic frits, including porous glass or quartz fritted
filters or porous glass or quartz fiber filters as well as ceramic
frits of various pore sizes. The zeolite suspension can be drawn
through the porous substrate with suction and the amount of zeolite
impregnated on the porous substrate will depend on the concentration
of zeolite in the suspension, the average pore size of the porous
substrate, the particle size of the zeolite material and the amount
of flow of the suspension through the substrate.
The pore size of the porous substrate will affect the characteristics
of the resulting membrane. Pore sizes varying between 1 and 250
microns can be acceptable and pore sizes of 2.5 to 50 microns are
more preferred.
To combine titania material with the substrate, the titania colloidal
suspension can be allowed to percolate through a zeolite impregnated
substrate with suction. The sol combined with the substrate is permitted
to dry by evaporation and leave the substrate containing a gel.
The substrate-zeolite-gel composite can then be fired under controlled
heating to yield a zeolite-titania membrane. An acceptable heating
rate is about 1.degree. C. per minute and acceptable firing temperatures
are between about 100.degree. C. and 900.degree. C., more preferably
between 400.degree. C. and 500.degree. C.
In an alternative process for forming a zeolite-titania membrane,
zeolite particles are suspended in the titania sol before it is
combined with the substrate. The zeolite-titania sol is then drawn
through the porous substrate to load the substrate with zeolite
material and titania material at the same time. This method can
be more efficient than when loading is performed as separate steps.
Another suitable method of incorporating both zeolite and titania
material into the substrate is to use dip coating. For example,
the zeolite particles are suspended in a titania sol and the porous
substrate is dipped into the zeolite-sol solution. The substrate
is then drawn out of the solution at a constant rate and permitted
to dry. Thereafter, it can be fired to yield a zeolite-titania membrane.
Still another method of forming a membrane in accordance with the
invention is the surface reaction method. For example, a zeolite-impregnated
substrate can be exposed to titanium isopropoxide or another titanium
alkoxide, in either the liquid or vapor state. The titanium alkoxide
will react with silanols (SiOH) on the surface of the porous substrate
and coat a film of titania on the surface of the substrate and promote
adhesion to the support.
For a liquid phase reaction, the porous substrates are first treated
with HCl or HF solution to produce surface silanols. Zeolite particles
(which can be pretreated with acid if desired) are then impregnated
in the substrate. The zeolite impregnated substrate is then put
into neat titanium isopropoxide or titanium alcohol solution. Reaction
with silanols takes place on the surface only because there is no
water or acid catalyst in the neat liquid or in the alcohol solution.
Zeolite-titania membranes are thereby formed on the substrate by
a surface reaction.
Zeolite impregnated substrates can also be exposed to titanium
isopropoxide vapor in a closed container. The vapor of titanium
isopropoxide reacts with the surface silanols to yield a film of
titania on the zeolite impregnated substrate to yield a zeolite-titania
membrane. Systems treated in this manner can be treated subsequently
with sols if desired. This combined approach can maximize the loading
of the titania matrix to both the zeolite and substrate surface.
Membranes in accordance with the invention exhibit excellent adsorptive
and ion exchange properties. Procedures for evaluating the adsorptive
properties of certain materials are described in Landolt, G. Analytic
Chem., Vol. 43 p. 6-13 (1971) and the procedures therein can be
used to test membranes in accordance with the invention. It was
thereby determined that zeolite-titania membranes in accordance
with the invention exhibit adsorptive capacities that are many times
greater than those of titania membranes, for the same weight of
material. Furthermore, when suspended in an aqueous solution of
cations, membranes such as NaA-titania membranes exhibit significant
ion exchange capacities.
Zeolite-titania membranes in accordance with the invention have
been shown to be particularly effective for use in photocatalytic
reactions. It is believed that the major deactivation pathway for
photocatalysis on a zeolite-titania membrane is the rapid recombination
of electrons and holes generated at the titania surface. The positively
charged holes can be responsible for the destruction of organic
contaminants. However, the photoefficiency of the photocatalyst
can be increased if the recombination process can be slowed or if
the electrons can be trapped. Thus, redox ions such as Cu.sup.2+
ions doped on the membranes can serve as reduction centers to quench
the free electrons. This increases the lifetime of the hole-electron
pair and increases the photocatalytic efficiency of membranes including
doped material.
To prepare a Cu.sup.2+ doped membrane, a typical molar composition
of starting material can be 1 Ti(OiPr).sub.4 :100 H.sub.2 O: 0.3
HCl . The weight ratio of Cu.sup.2+ to the titanium dioxide formed
in the hydrolysis can be in the range of 0.1% to 20%, more preferably
in the range of from 1% to 10%. The resulting membranes will have
photoefficiencies even higher than those of the nondoped zeolite-titania
membranes.
Zeolite titania membranes in accordance with the invention can
be used to catalyze many important chemical reactions. For example,
they can mineralize toxic organic compounds into innocuous products.
Chemicals such as trichloroethylene (TCE), chloroform and benzene
can be mineralized to carbon dioxide, hydrochloric acid and water
in the presence of membranes in accordance with the invention such
as silicalite-titania membranes, Cu.sup.2+ doped silicalite-titania
membranes or ETS-10-titania membranes when the membranes are illuminated
with appropriate wavelength electromagnetic radiation.
As noted above, the molecular sieve-titania membranes of the invention
have a relatively high adsorptive capability, compared to conventional
titania membranes. Thus, they can adsorb a wider range of organic
molecules and can hold a larger number of molecules longer and more
tightly. This increases the probability of reactive collisions between
the active sites and the organic molecules or electron transfer
through molecular sieve channels. The relatively long retention
time of molecules on the membrane leads to an increase in the photoefficiency
(quantum yield) of the membrane. Quantum yield can be defined as
the moles of molecules converted per moles of photons adsorbed.
Quantum yield can be calculated from the intensity of incident light,
the area of membrane exposed and the observed yields of the reactions.
For example, a three to five-fold increase in quantum yield has
been observed in the photocatalytic mineralization of toxic organic
contaminants using a zeolite-titania membrane, compared to the same
reaction using a conventional titania-slurry. An additional two-fold
increase can be obtained by doping the membrane with metal ions.
The properties of the impregnated zeolites in the membrane can
influence the photoefficiency of the membrane. This is especially
true when the molecular sieve-titania membrane is used in the treatment
of contaminated water. The hydrophilicity of a zeolite depends on
its Si/Al ratio. Lowering the ratio can make the zeolite more hydrophilic.
Thus, when zeolite-titania membranes are used to mineralize organic
chemicals present in water, the water molecules compete with the
organic molecules for the adsorptive sites. The amount of water
molecules absorbed by the impregnated zeolite will increase as the
Si/Al ratio is decreased. Accordingly, silicalite, which has little
aluminum, adsorbs very little water and leaves most of the adsorptive
sites available for incorporating organic molecules. In contrast,
ZSM-5 which has a relatively low Si/Al ratio absorbs water from
the aqueous solution and leaves relatively few adsorptive sites
for the organic molecules. Accordingly, the photoefficiency of a
zeolite-titania membrane will increase with the increase of the
Si/Al ratio.
ETS-10 is a titanium containing molecular sieve material. The synthesis
and certain properties of ETS-10 are described in U.S. Pat. No.
4853202 the contents of which are incorporated herein by reference.
ETS-10 has been determined to have a high adsorptive capability
and contains titanium sites. It is thus a preferred material for
use in a molecular sieve-titania membrane for mineralizing toxic
organic compounds.
Molecular sieve-titania membranes can catalyze other reactions
as well. For example, water can act as an electron scavenger and
carbon monoxide can serve as a hole scavenger and a molecular sieve-titania
membrane can catalyze the water-gas shift reaction. For example,
when doped with platinum, a molecular sieve-titania membrane can
catalyze the splitting of water to generate hydrogen fuel and mimic
photosynthesis. It can also convert biomass such as proteins, fats
and carbohydrates to H.sub.2 fuels and CO.sub.2.
Photocatalytic reactions involving membranes are preferred to be
carried out in two types of reactors. A first type is a flow reactor,
in which the reactants flow through a reaction vessel, at rates
which can typically range from 0.6 ml/min/cm.sup.2 to 1000 ml/min/cm.sup.2
through a molecular sieve-titania membrane that is illuminated by
a lamp of appropriate wavelength. Another type of reactor is known
as a steady-state reactor, in which the molecular sieve-titania
membrane is suspended in a stirred solution and illuminated by a
lamp.
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